Project Summary Melanoma is a highly aggressive cancer with a rising incidence and good therapeutic options exist for only a subset of patients. Recent studies have shown that treatment with an autophagy inhibitor can improve outcome for melanoma patients. Recognizing the importance of interactions between cancer cells and the host, we chose to investigate the role of autophagy in the stroma. We discovered a new connection between the metabolic/autophagic state of the host, and the immune response to a tumor. We found that the host fibroblast-rich stroma adjacent to a melanoma have higher levels of autophagy marker LC3 than cells distant from the tumor. Using co-culture models, we discovered that cancer cells induce autophagy in co-cultured fibroblasts in a TGF-?-dependent manner. Orthotopic melanoma allografts showed dramatic growth reduction in genetically-engineered mice in which autophagy protein Atg7 was inactivated in the entire animal, or only in fibroblast specific protein 1 (FSP1)- expressing cells, which includes fibroblasts and immune cells. Tumors grown in Atg7-deficient hosts had lower levels of proliferation and higher levels of apotosis than tumors hosted in control mice. We conclude that Atg7 expression in the host plays an important role in supporting tumor progression. To understand the mechanism through which host Atg7 inactivation affects tumor growth, we performed RNA-Seq analysis of tumors harbored in Atg7-inactivated and control mice and discovered a gene expression signature of inflammation. Analysis of cytokines in the plasma and T cells of Atg7-deficient mice revealed higher levels of Th1 type cytokines and no change in Th2 type cytokines. Memory CD4+ and CD8+ T cells from Atg7-deficient mice transitioned from nave to effector states. Tumor infiltrating lymphocytes in Atg7-deficient mice contained higher levels of CD8+ T cells than in control mice. Treatment with antibodies that deplete T cells resulted in larger tumors in Atg7-deficient mice when tested in the whole body conditional knockout model or in mice with FSP1-specific autophagy inactivation. We have therefore discovered a new mechanism regulating the immune response to tumors. We will investigate the role for additional cell types in anti-tumor immunity in Atg7-deficient mice, and the role of Atg7 in anti-tumor and anti-antigen effector responses. We will test whether small molecule autophagy inhibitors also induce anti-tumor immunity and whether autophagy inactivation sensitizes mice to immune checkpoint inhibitors. We will also test whether the levels of host autophagy in specific cell types correlates with the extent of immune cell infiltration into melanomas in patient samples. We anticipate our findings will suggest new approaches for activating a patient's immune response to fight cancer.